T cell activation in response to MHC-bound antigen is dependent upon co- stimulatory signals from antigen presenting cells in addition to the interaction of the T cell receptor with the MHC-antigen complex. In vitro, interruption of either the B7/CD28 or the gp/CD40 co-stimulation pathway can result in antigen-specific unresponsiveness. While the ability to induce antigen-specific tolerance in vivo would have broad therapeutic implications in humans, attempts to induce long-term tolerance by interruption of these pathways in vivo has produced mixed results. We have recently observed that while blockade of either the B7/CD28 or the gp39/CD40 co-stimulatory pathways result in suppression of primary immune responses in vivo, simultaneous blockade of both pathways results in sustained antigen-specific tolerance to a soluble protein antigen in normal mice. Based on these observations, the proposed studies will clarify the mechanism underlying this effect. Specifically, we will determine whether interruption of signaling by specific BY molecules is necessary for tolerance induction, whether tolerance induction is accompanied by changes in T cell cytokine production, and whether this effect applies to other potent immunogens, including monoclonal antibodies. We will also determine whether interruption of co-stimulation during primary antigen exposure is necessary for the induction of tolerance, or whether antigen-specific tolerance can also be induced after antigen priming has occurred. Finally, the proposed studies will determine whether interruption of B7/CD28 and gp39/CD40 co-stimulation can result in antigen-specific unresponsiveness in NZB/NZW lupus-prone mice, a strain that is particularly resistant to tolerance induction. These studies will provide the basis for future proposals aimed at inducing tolerance to auto-antigens in autoimmune mice.